Pseudorandom assignment between elements of the image processor and the A/D converter cells

ABSTRACT

An image sensor with a plurality of elements which received images and a plurality of A/D conversion elements. A connection between the A/D converter and the image elements is substantially randomly assigned to avoid fixed pattern noise.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is a continuation of U.S. application Ser. No.10/199,955, filed on Jul. 19, 2002, now U.S. Pat. No. 6,787,752, thedisclosure of which is herewith incorporated by reference in itsentirety.

This application claims benefit of U.S. Provisional Application No.60/306,753, filed Jul. 19, 2001.

BACKGROUND

An image acquisition system typically includes an array ofphotosensitive pixels which are successively switched in a circuit toconnect to A/D converter cells. The photosensitive pixels receiveincoming light, and produce an analog output indicative of theinformation in each pixel in the array. That analog information is thencoupled to an analog to digital converter which converts the analoginformation into a digital value indicative thereof. Typically theelements of the array are connected to different analog to digitalconverter elements. The pixels are successively converted into digitalvalues.

One way of carrying out this analog to digital conversion is viasuccessive approximation A/D converter cells. However, the variationbetween such successive approximation cells causes an inherent variationin the output signals. This can cause fixed pattern noise, that is, anoise pattern which is superimposed over the actual image beingobtained.

SUMMARY

The present invention describes a technique which may minimize the fixedpattern noise in a system which uses multiple A/D converters along withan array of image acquisition elements.

An aspect of this system may randomly assign relations between the imageacquisition elements and the A/D converters.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other aspects will now be described in detail with referenceto the accompanying drawing, in which:

FIG. 1 shows a block diagram of a system which dynamically assignsbetween image sensing elements and A/D converters.

DETAILED DESCRIPTION

A typical system which uses multiple A/D converters operates as follows.The image sensor array is connected to switches which switch pixels ofthe image sensor array to analog to digital converters in a specifiedsequence. For example, there may be multiple analog to digitalconverters, especially when successive approximation type converters areused. Each of these converters may require multiple clock cycles to“settle” on a specified digital value. Therefore, multiple convertersare often used to increase the throughput of the system as a whole. In atypical image sensor device, the pixels of the array are switched to theconverters in sequence. For example, the first pixel in row 1 may beswitched to the first of the successive approximation converters. Thesecond pixel in row 1 may be switched to the second of the convertersduring the same time period. During a subsequent clock cycle, the secondrow is converted in a similar way; with the first pixel in the rowswitched to the first converter and the second pixel in the row switchedto the second converter.

The inventor noticed, however, that this requires that each successiveapproximation cell is always used for the same conversion within aframe. Any variation between these cells forms a fixed pattern of noise.This noise can be seen by a user.

According to the present invention, a circuit is described whichprovides a pseudo random assignment between successive approximationcells and sensors in the image. In the embodiment, there are N of theA/D converter cells, which in the embodiment can be successiveapproximation A/D converters. Four cells, 140, 142, 144, 146 are shown.In a particularly-preferred embodiment, there may be the same number ofcells as there are elements in a particular row of the image sensor. Theimage sensor 100 is shown with a first row 104 and a second row 106,each having also four elements. In an exemplary system, each column isswitched, such that its pixels go to a specified A/D converter. In thisembodiment, each of the pixels 102 is connected to 1×N switching circuit120. The 1×N switching circuit 120 has the capability of switching anyof its inputs 122 to any of its outputs 124. The switching arrangementof the 1×N switching circuit 120 is controlled by a random numbergenerator 130, which produces a random number output 126.

By random, applicants intend to cover all forms of numbers which areeffectively random, such as pseudorandom, and other such sequences thatcontinually change. That is, the random number generator 130 will outputa constantly varying stream of digits, which may be pseudorandom, butwhich will be continually varying. The effect is that each time an imagesensor element is read out, it is randomly, or effectively randomlyassigned to one of the successive approximation cells.

The output values are connected to an image processor 150 which imageprocesses the output in the conventional way. For example, this may usecorrelated double sampling and the like.

The effect of this system is that even though there will still be thesame amount of variation between successive approximation cells, therewill not be fixed pattern noise. The noise is still at the same level.However, the noise is spread randomly among the cells, and hence thehuman eyes will be much less sensitive to that noise. The random noise,in a perfect situation, would effectively average out. Therefore, theeffect of the noise will be much less pronounced than the effect of thenoise in other such systems.

The combinations may randomly map between the pixels and the nsuccessive approximation cells. For example, if there are four of thesuccessive approximation cells, the possible combinations may be (1, 3,0, 2); (3, 0, 2, 1); (0, 1, 3, 2) and the like. There are severaldifferent techniques available to make these kinds of combinations.

It is also possible to have M (=N/n) combinations in order to simplifythe generator, in which case the generator may extend between 0 andM−1+k*M.

Any other technique of forming a pseudorandom association may also beused as long as the device effectively continuously varies therelationship between the pixels and the A/D converter cells whichconvert the pixels.

Although only a few embodiments have been disclosed in detail above,other modifications are possible. For example, other kinds of A/Dconverters may be used in the same way, and this technique is applicableso long as there is more than one A/D converter. Other techniques besidepseudorandom number generators can be used to continuously vary theconnection between the pixels and the A/D converter elements. Inaddition, this system can operate on a line-by-line basis, anelement-by-element basis, or a frame-by-frame basis. For example, theconnections between image sensor element and A/D converters need not bedone more often than once per frame, if necessary. All suchmodifications are intended to be encompassed within the followingclaims:

1. An image processing system, said image processing system comprising:an image sensor, said image sensor comprising: an image acquisitionarray, having an array of image acquisition elements; a plurality of A/Dconverter elements, each of said A/D converter elements capable ofconverting a signal from a single element of said image sensor arrayinto a digital value; a switch array, between said array of imageacquisition elements and said A/D converter elements, said switch arraysubstantially randomly varying an assignment between each of said imageacquisition elements and each of said A/D converter elements; and animage processor, said image processor processing an image acquired bysaid image sensor.
 2. The image processing system of claim 1, furthercomprising a random number generator, which produces a number which iseffectively random, and outputs said number to said switch array.
 3. Theimage processing system of claim 2, wherein said random number generatoris a pseudorandom combination generator.
 4. The image processing systemof claim 2, wherein said A/D converter elements are successiveapproximation elements.
 5. The image processing system of claim 2,wherein said image acquisition elements are CMOS image sensing elements.6. The image processing system of claim 2, further comprising acorrelated double sampling element, receiving an output of each of saidA/D converter elements.
 7. A method of processing an image, comprising:receiving image elements from pixels of an image sensor; substantiallyrandomly assigning each element from each pixel to a specified A/Dconverter for conversion; and processing said image acquired by saidimage sensor.
 8. The method of claim 7, wherein said substantiallyrandomly assigning comprises assigning using a pseudorandom combinationgenerator.
 9. The method of claim 7, further comprising A/D convertingsaid image signals from said pixels in the A/D converter which isassigned substantially randomly.
 10. The method of claim 9, wherein saidA/D converting comprises using the successive approximation technique toA/D convert said image signals.
 11. The method of claim 7, wherein saidsubstantially randomly assigning comprises using a switch to switchelements from said pixel to elements from said A/D converter.
 12. Themethod of claim 11, wherein said substantially randomly assigningfurther comprises driving said switch using a pseudorandom combinationgenerator.
 13. A method of processing an image, comprising: obtainingimage elements in pixels of an image sensor; substantially randomlyassigning each element to an A/D converter; converting said imageelements in a plurality of separate image A/D converters which havevariations between the A/D converters but without adding fixed patternnoise via said converting; and processing said image acquired by saidimage sensor.
 14. The method of claim 13, wherein said convertingcomprises substantially continually varying a connection between saidpixels and said A/D converters.
 15. The method of claim 13, wherein saidconverting comprises using a pseudorandom number generator to assign arelationship between said image elements and said A/D converters. 16.The method of claim 13, wherein said converting comprises successiveapproximation A/D converting.
 17. The method of claim 13, wherein saidobtaining image elements comprises using a CMOS image sensor to obtainsaid image elements.
 18. An image system, comprising: an imageacquisition array, having an array of image acquisition elements; aplurality of A/D converter elements; a random output element, whichproduces a random output, coupled to said image acquisition array andsaid plurality of A/D converter elements; and a processor whichprocesses an image acquired by said array.
 19. The image system of claim18, wherein said random output element produces a random number.
 20. Theimage system of claim 19, further comprising a switch between said imageacquisition array and said A/D converter elements, and wherein saidrandom number sets a connection of said switch.
 21. The image system ofclaim 20, wherein said switch is a 1×N switch, where N is a number ofA/D converter elements.
 22. The image system of claim 21, wherein saidnumber sets a connection between each of said inputs, and each of saidA/D converter elements.
 23. The image system of claim 21, wherein saidA/D converter elements are successive approximation A/D converterelements.
 24. The image system of claim 18, wherein said random outputelement is a random number generator that controls the connectionbetween elements of said image acquisition array, and said A/D converterelements.